The present invention relates generally to developing systems for electrostatic latent images. More particularly, the present invention relates to a developing system in which mono or one-component a developer supplied onto a developer sleeve is formed into a developer layer having a predetermined thickness and a predetermined amount of electric charge by a restriction member, and the developer layer is transferred to an electrostatic latent image to make the latent image visible.
In the area of dry-type developing systems for developing electrostatic latent images formed on electrostatic latent image retainers, a binary or two-component developing system has been developed and put into practical use which uses a two-component developer containing toner particles and carrier particles. The toner particles are charged by triboelectrification with the carrier particles, so that the charged toner particles are electrostatically attracted to an electrostatic latent image. This developing system has proven satisfactory in practical use. However, recently a one-component developing method and system, which uses a one-component developer containing only toner particles, has been proposed to avoid problems in deterioration of the developer.
Such a one-component developing system may be classified into two types, one using a magnetic developer and the other using a nonmagnetic developer. In addition, a mono component system may develop the image by direct contact between the developer and the electrostatic latent image, or without any such direct contact.
An example of the one-component developing system is disclosed in Japanese Patent Unexamined Publication No. 53975/85 published on Mar. 28, 1985. This example is shown in FIG. 1, herein. In the drawing, the reference numeral 1 designates an electrostatic latent image retainer which retains on its surface an electrostatic latent image 2 with a surface potential of 200 to 900 V. A developer sleeve 4 is made of phenol, including carbon, glass fiber, and the like, and has a thickness of 1 mm, a specific resistance of 1.times.10.sup.10 .OMEGA.cm and a relative dielectric constant .epsilon.=30. This sleeve 4 is disposed on a developer electrode 3 in opposition to the surface of the electrostatic latent image retainer 1 at a distance of about 100 to 400 .mu.m. The surface of the developer sleeve 4 is polished in the axial direction (that is, in the direction perpendicular to the direction of carrying a developer) with sandpaper or the like so as to provide a surface roughness (Rz: average roughness in 10 measurements in accordance with the Japanese Industrial Standard) of about 1 to 10 .mu.m.
A supply member with a depth of about 1 mm for supplying a developer 5, reserved in a hopper 10, onto the developer sleeve 4 is urged against the developer sleeve 4 so as to rotate in the direction of the arrow in the drawing at the same peripheral velocity as that of the developer sleeve 4. In order to reduce the load exerted on the developer sleeve 4 when the supply member 6 is urged against the sleeve 4, a shaft 8 is used as the supply member 6. A foamer 9 of urethane or the like is wound around the shaft 8 and the shaft is further covered with EPDM rubber (ethylene propylene rubber) or the like in a thickness of 0.5 to 1 mm. On the surface of the rubber, convexities or projection are distributed equidistantly at 15.degree.with a height of about 200 m, to thereby transfer the developer 5 between the convexities onto the developer sleeve 4. The thus transferred developer 5 is transported to a restriction member 7 by the rotation (in the direction of arrow in the drawing) of the developer sleeve 4, and is electrically charged by triboelectrification. The charged developer 5 is formed into a uniform layer (0.3 to 1.0 mg/cm.sup.2) on the developer sleeve 4, so as to be transferred onto the electrostatic latent image 2.
The restriction member 7 comprises a 0.1 mm thick spring material of SUS304 CSP3/4H, and a 1 mm thick silicone rubber material of rubber hardness 50.degree., the silicone rubber material containing an additive filler of silicone dioxide and titanium dioxide. The contact pressure between the restriction member 7 and the developer sleeve 4 is set to a value within a range from 50 to 300 g/cm.
A high-frequency alternating voltage having a frequency of 1-10 KHz and Vpp of 400-4500 V, on which a DC voltage of 200-400 V is superimposed, is impressed on the developing electrode 3 to form a peripheral electric field at the electrostatic latent image portion 2 on the electrostatic latent image retainer 1, to thereby move the developer 5 and cause development.
The developer 5 is a nonmagnetic one-component developer having a particle diameter of 5-20 .mu.m. It is prepared by dispersing a pigment, such as carbon black or the like, and a polarity control material, such as a metal-including dye or the like, in various kinds of thermoplastic resins, such as a styrene resin, an acrylic resin, etc., and grinding and classifying the same. When occasion demands, hydrophobic silica may be added to the developer 5 in an amount within a range of from 0.1 to 2.0% by weight per amount of the particles of the developer 5, in order to improve movability of the developer 5 from the developer sleeve 4 onto the electrostatic latent image 2.
In the conventional one-component developing method as described above, uniformity of the layer of the developer 5 after passing through the restriction member 7 has a large influence on picture quality. In other words, when a uniform layer of the developer 5 is prepared, it is possible to obtain a distinct picture having a uniform and dark image in the solid region and no fog in the background region. On the contrary when the layer of the developer 5 is not uniform, only a poor picture which has an ill-balanced and light image in the solid part and fog in the background region is obtained.
In the prior art, the layer of the developer 5 has been uniform on some occasions, but it also has been ill-balanced and non-uniform at other times. Accordingly, a uniform layer of the developer 5 could not be prepared consistently so that only a poor picture having an ill-balanced and light image in the solid region and fog in the background region has been obtained.
Accordingly, it is an object of the present invention is to solve the problems in the above-mentioned developing system, or in other words to provide a consistently uniform developer layer on a developer sleeve so as to provide a copy having a uniformly clear image in the solid region and no fog in the background region.
Additional objects and advantages impact will be obvious from the description, and in part may be learned by practice of the invention.